He’s also work(ed) with other technologies such as React, React Native, Golang, Python, PHP and the list goes on.
But he doesn’t plan to work with those professionally nor specialize in them so, don’t trouble to ask.

Learning

Erlang — Erlang is the underlying language of Elixir and many stuff on Elixir still uses
concepts and libraries written in Erlang, therefore he’s accepted the self-purposed endeavour of learning it as well;

BEAM — BEAM is the the virtual machine where Elixir/Erlang code runs on
so it’s also an interesting topic to go deeper;

OTP — Last but not least, the Open Telecom Platform (OTP). It’s a set of
modules and libraries used by most Elixir/Erlang applications to take advantage of the awesome capabilities those offer in terms of processes management, concurrency, etc. in a simple way without having to re-invent the wheel.